The present invention relates generally to the production of coal in situ into combustible gases, synthetic crude oils, coal chemicals and an underground system for production of industrial steam.
The civilized world is highly dependent on sources of energy for the necessities and amenities of life. In early times wood provided the energy for heat and light. With a growing world population and with forests denuded around the populated areas, coal gained favor as a source of heat and light, and later provided a source of energy for mechanized transportation and a host of other mechanical devices. Coal, of course, is more compact than wood and, therefore, contains more energy per unit weight or unit volume, and from that point of view is more desirable than wood.
As sources of energy, both wood and coal involve a series of batch operations. For wood, the tree is found and felled, useless parts such as twigs and leaves separated and disposed of, then lengths are cut to appropriate sizes, loaded on conveyances, carted to the point of use, off-loaded, stacked, picked up a few pieces at a time and cast into the fire, ashes are then removed and disposed of, and so on. Similarly, coal is found, grubbed out, obvious extraneous matter separated and disposed of, then broken down or crushed to desired sizes, loaded, transported to the point of use, off-loaded, piled, picked up and cast into the fire, then ashes and clinkers are removed and disposed of, and so on.
The discovery of commercial quantities of crude oil and natural gas led to massive displacements of wood and coal as sources of energy. Petroleum, of course, compared to wood or coal contains more energy per unit weight. Petroleum is fluid, clinker free, and is or can be made ash free. Further, petroleum can serve as a source of energy in a series of continuous operations from the oil field to the end use. Batch operations, by nature costly, are essentially eliminated and messy cleanup as an aftermath of use is also eliminated. For decades petroleum discoveries were so prolific that supplies substantially exceeded demands with resultant abnormally low prices compared to other commodities in commerce.
Like the denuded forest of old, times today have changed. The easy to find oil fields of the world have been found. New discoveries of oil fields in recent years have tended to be located vast distances from population centers. The laws of supply and demand have been supplanted with international politics in the setting of market prices. Thus coal has been reinstated as a major source of future energy supplies.
Coal has retained its advantages of being more favorably located in relation to the population centers of the world. Worldwide reserves of coal dwarf the known worldwide reserves of petroleum. For almost 100 years petroleum has been available in copious quantities at abnormally low prices. As a consequence, worldwide technical development was focused on petroleum to the virtual exclusion of technical development in coal. A look at the coal industry today reveals only token improvements over the old batch operations of grub, sort, crush, load, cart, off-load pile, pick up, stoke and clean up. While it is true that individual operations have become highly mechanized with mammoth devices, the elements of batch operations remain. Batch operations, no matter what size, have great difficulty in competing with continuous operations of similar size.
The state of the art in the coal industry requires a lot of catching up to match the state of the art in the petroleum industry. First, coal should be brought to the surface as a fluid. A review of the prior art in coal shows that most of the work to fluidize coal has been performed after the coal was brought to the surface as a solid. This arrangement, of course, retains the batch operations of grub, sort, crush, load, cart, off-load, pile and pick up. After these batch operations have been performed and coal is transported to suitable above ground pressure vessels, it is well known in the art how to fluidize coal into combustible gases, into coal chemicals, and into synthetic crude oil. Unfortunately these operations also tend to be batch or semi-batch types.
Since the preponderance of the prior art of the above ground fluidization of coal begins after the coal has been mined by conventional methods, the feedstock is delivered with its two principal impurities -- moisture and ash contents -- intact. Moisture may be substantially removed in a separate batch operation, but the ash content is normally introduced into the pressure vessel for removal at a later step in the fluidizing process. It should be obvious that a vast improvement would be made if the moisture content and the ash content were separated before the coal is brought to the surface.
Some prior art has dealt with fluidizing coal in situ. The preponderance of this work has been involved with in situ gasification of coal with the objective of producing combustible gases. Large scale operations were undertaken in Russia with lesser projects of shorter duration undertaken in the United States, England, Morrocco and other localities All have been plagued with problems of underground burning consuming the combustible gases before they could be delivered to the surface. All have produced low BTU gases (in the range of 85 to 300 BTU per standard cubic foot) compared to natural gas of petroleum origin containing approximately 1000 BTU per standard cubic foot. These low BTU gases, while not suited to long distance pipelining, are quite satisfactory for nearby use if the BTU content can be stabilized at a reasonably constant level.
All in situ gasification projects heretofore seem to have overlooked a significant fact in their quest to generate combustible gases. The purpose of combustible gases as fuel is to generate heat. It, therefore, follows that it may not make too much difference whether the gas is burned below ground or above ground as long as the heat is captured to perform the useful work intended. If the heat is captured underground and brought to the surface, then the bothersome problem of preventing unplanned burning of combustible gases underground is eliminated. Methods of capturing heat underground will be apparent later in this disclosure.
A search of the prior art has revealed a meager amount of meaningful work in attempting to subject coal to pyrolysis in situ. Methods of pyrolizing coal in situ will be apparent later in this disclosure.
There has been a limited amount of work in the art of in situ liquefaction of coal. Methods have been described in U.S. Pat. No. 2,595,979 of Pevere et al, beginning with coal at ambient temperatures. No projects are known to applicant where coal has been liquefied in situ, using coal that is already hot. Methods of liquefying coal in situ, using hot coal as the raw material, will become more apparent later.
In order to understand the problems of producing coal in situ, it is helpful to understand some of the characteristics of coal. Coal had its origin in ancient geological times when large areas of the earth were relatively flat and swampy, and plant life grew in profusion. Over and over plants sprouted, grew, matured, died, fell in the water, then were replaced by many generations of other plants which repeated the cycle. Severe rotting occurred to dead plant parts protruding above the water, while submerged plant parts were substantially preserved. The accumulated plant debris, often many feet thick, contained a variety of components including roots, trunks, bark, limbs, leaves, moss, reeds, grasses, and mineral matter deposited by dust laden winds. Later in geological time the areas were inundated and deposits of mud, sands and clays sank to the bottom. These sediments ultimately formed the shales, sandstones, and limestones that overlie coal deposits today. The sediments, of course, provided the weight to compact the plant debris and thus began the evolution into coal. With the variety in the plant debris it is easy to understand why today some coal is hard, some soft, some difficult to crush, some easy to crush, some highly permeable, some with hardly any permeability, and so on. With buckling of the earth's crust, such as occurred when mountains were formed or during earthquakes, it is also easy to understand how some coal deposits underground contain an extensive pattern of fractures and cracks that permit the passage of fluids.
For purposes of illustration, subbituminous coals as found in the western part of the United States are used in describing the processes herein, although coals of higher or lower rank are also applicable. These coals contain carbon, hydrogen, moisture and mineral matter. The carbon and hydrogen are combined into hydrocarbons that are similar to those found in crude petroleum, although the total hydrogen content in coal is only about half that of similar units of crude petroleum. It is this hydrogen deficiency in coal compared to petroleum, that prevents coal from being a ready substitute for petroleum. A proper planning of processes and projects, as will be described hereinafter, can produce products from coal that are readily interchangeable with products from crude petroleum.
The most prevalent use of hydrocarbons is as a fuel, whether the source be from petroleum or coal. In the combination process hydrogen (H.sub.2) is burned with oxygen (O.sub.2) to form water vapor (H.sub.2 O), carbon is burned with oxygen to form carbon dioxide (CO.sub.2), and any sulfur present forms sulfur dioxide (SO.sub.2). These are the reactions when there is sufficient oxygen present to yield an oxidizing environment. With a shortage of oxygen and thus a reducing environment, substantially all of the carbon burns to carbon monoxide (CO) and sulfur combines to form hydrogen sulfide (H.sub.2 S). In the combustion zone it is possible to have both oxidizing and reducing environments which will result in products of combustion containing water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen sulfide, free hydrogen, free oxygen and free carbon. As a practical matter in commercial operations it is desirable to control combustion either to a predominantly oxidizing or to a predominantly reducing environment.
In an oxidizing environment, the water vapor and carbon dioxide have contributed the maximum to the generation of heat from the fire. The sulfur dioxide can be further oxidized with a catalyst into sulfur trioxide (SO.sub.3) which combines with water vapor to form a sulfuric acid mist (H.sub.2 SO.sub.4). Thus the oxidizing environment yields the most heat but in the presence of sulfur yields objectionable sulfur dioxide, sulfur trioxide or sulfuric acid, all of which are troublesome in the exit gases.
In the reducing environment, the carbon monoxide that is produced can be further oxidized and thus has a useful calorific content (approximately 315 BTU/cu ft) as a pipeline gas. The presence of sulfur yields hydrogen sulfide, which is relatively simple to separate from the exit gases. The reducing environment generates substantial quantities of heat, but much less than the oxidizing environment. In the predominantly reducing environment carbon dioxide (CO.sub.2) reacts with incandescent carbon to form additional carbon monoxide (CO). As is well known in the art practiced above ground, incandescent carbon in the presence of water (or steam) reacts to form producer gas as follows: EQU H.sub.2 O + C = H.sub.2 + CO
this reaction absorbs considerable heat, but at the same time releases two valuable gases, hydrogen and carbon monoxide. Both of these gases, when properly redirected as described herein, server as feedstocks to upgrade nearby coal in situ. The hydrogen generated underground is particularly useful in remedying the hydrogen deficiency of a portion of the coal in situ and also can be used as a feedstock for commercial facilities above ground.
A survey of the coal research and development shows that the preponderance of effort is directed to work above ground in gasification and liquefaction. All projects are plagued with a common problem; the hydrogen deficiency of coal. To understand the magnitude of the problem, consider the manufacture of fuel gases from coal. As previously mentioned, it is well known in the art how to derive producer gas (sometimes called blue water gas) by reacting steam with incandescent carbon to form hydrogen and carbon monoxide. Both hydrogen and carbon monoxide are good fuel gases, each containing slightly over 300 BTU per cubic foot. Both fall woefully short in heat values; however, when compared to natural gas of petroleum origin which contains approximately 1000 BTU per cubic foot. It is well known in the art how to upgrade producer gas into gases with higher BTU content, but if upgrading is expected to be compatible with natural gas (principally methane, CH.sub.4), makeup hydrogen is required in substantial quantities. For a typical coal to be upgraded into methane, almost three times as much hydrogen is required as is contained in the original coal. For liquefaction of coal, makeup hydrogen is also required because synthetic crude oil from coal contains approximately twice as much hydrogen as the original coal contained. Coal chemicals, however, can be extracted from raw coal without makeup hydrogen, simply by subjecting the coal to heat in the absence of air and capturing expelled gases and oozing tars.
Most underground coal deposits contain a certain amount of trapped gas in the pore space and in channels of permeability. The most common entrained gas is methane (sometimes called fire damp) which often is found in quantities of 50 to 300 standard cubic feet per ton of coal in place. This gas is a fire hazard and a health hazard to underground workmen. Since the processes described herein require no manpower underground, entrained methane is readily captured for commercial use.
Referring again to producer gas generated from coal, either above ground or in situ, it is easy to understand the commercial desirability of upgrading. First is the problem of transportation. Cross country pipelines experience about the same amount of costs whether the gas transported be producer gas at 320 BTU per cubic foot or natural gas at 1000 BTU per cubic foot. It, therefore, follows that a million BTU's of producer gas at the destination will cost approximately three times as much in transportation charges as the same amount of BTU's delivered as natural gas. Second, while producer gas is an excellent fuel, it is not compatible with natural gas at the burner tip. Heating devices must be designed for one or the other, and substantial mechanical modifications normally must be made to convert from one gas to another.
With the worldwide reawakening to the importance of coal as a source of energy, both as a direct source of fuel and as a source of feedstocks for synthetic fuels, considerable outcry has been advanced regarding the environmental impact of coal production. In the United States, for example, powerful lobbying groups have joined forces to stop or severely restrict some of the mining methods practiced in the past. Gutting of the countryside, no doubt, will be a practice of the past, both in the United States and elsewhere. Coal production operations of the future must be designed to minimize damage to the environment as well as provide for restoration to proper aesthetic values upon termination of operations. Gutting of the countryside, in itself a costly operation, is overshadowed in terms of cost by the effort required in restoration. Restoration, no matter how well planned, leads to virtually endless differences of opinions as to the effectiveness of the job.
A minimum environmental impact occurs when coal is consumed in situ. Surface disturbance is kept to a minimum by drilling wells into the coal deposit. Then the coal can be subjected to in situ gasification, pyrolysis and liquefaction. By proper planning, subsidence can be controlled over a wide area, resulting in minor lowering of the landscape, the surface of which remains virtually intact.